The present invention relates generally to high-energy electrochemical cells, such as lithium-based cells, other secondary cells, and batteries constructed therefrom. More particularly, the present invention relates to systems and methods for characterizing electrochemical cells and for predicting the response of such cells to thermal, mechanical or electrical abuse based on power functions obtained from calorimetry.
Rechargeable electrochemical cells are currently used to power a wide variety of portable electronic devices, including laptop computers, cell phones, cameras, and personal organizers, for example. The increased use of such mobile devices has placed a greater demand on the battery manufacturing industry to provide high powered cells that may be used safely in a wide spectrum of consumer and industrial applications. In order to minimize size and weight, battery technologies with high-energy density are normally used. Larger versions of such technologies may, for example, be used in hybrid or all-electric vehicles. High-energy density cells store large amounts of energy in relatively small volumes. If this energy is released quickly and in an uncontrolled manner, however, thermal runaway is possible, leading to safety concerns.
Lithium-ion and lithium-ion polymer cells (collectively referred to as lithium-ion cells in the following discussion), for example, exhibit the largest energy density of all ambient-temperature rechargeable cell technologies. Lithium-ion cells are carefully engineered to meet a variety of safety test standards, including, for example, UL-1642 (Underwriters Laboratories) and IEC-61960 (International Electrotechnical Commission) standards. The tests defined by these standards include oven exposure, short-circuit, forced overcharge, forced discharge, shock and vibration. Other proposed tests include nail penetration tests. It is desirable that cells and batteries constructed from such cells do not emit smoke or flame when subjected to thermal, electrical, and mechanical stress associated with the above-identified tests.
In addition to the electrical energy which lithium-ion cells can deliver during discharge, these and other high-energy cells can also evolve a considerable amount of heat due to the reaction of the electrode materials with the electrolyte. During short-circuiting of a cell, for example, both the electrical energy of the cell and the chemical heat resulting from the electrode/electrolyte reactions are dissipated as heat within the cell. Thermal runaway can occur if the sum of these thermal powers is greater than the power that can be transported from the cell to the environment.
The UL and IEC oven exposure tests probe the severity of electrode/electrolyte reactions. These reactions are most severe when the cell is fully charged. In accordance with these oven exposure tests, a fully charged cell is placed into an oven and exposed to a temperature of 150xc2x0 C. (UL) or 130xc2x0 C. (IEC) for a predetermined duration of time. Short-circuiting of the cell under test normally does not occur and the cell temperature rises above the oven temperature to the point where the power generated by electrode/electrolyte reactions is equal to the power that can be transferred to the environment. However, if the former is always larger than the latter, thermal runaway occurs. It is noted that, although cells in consumer use are typically not placed in ovens at high temperature, they may be exposed to 85xc2x0 C. environments in battery cases that inadvertently are thermally well-insulated. If electrode/electrolyte reactions proceed significantly at such temperatures, insulated batteries could exhibit thermal runaway.
The total power generated by the electrode/electrolyte reactions (under a specific set of circumstances) is proportional to the total volume of the cell. That is, if two cells have the same chemistry, the same construction details and the same charging history, but one has twice the volume of the other, then the larger cell will evolve twice the power due to electrolyte/electrode reactions at elevated temperature than the smaller one. The power that can be transferred to the environment, however, is proportional to the cell surface area. Therefore, it is expected that the cell surface area to volume ratio should be maximized to optimize cell safety. This is not always possible due to cell manufacturing constraints or physical size limitations of a device within which the cell will be housed.
Given the issues discussed above, it can be appreciated that cell designers are faced with a complex task. The cell designer is often asked to maximize cell performance, cell energy density, and cell safety. Design changes that maximize energy density may, however, adversely compromise safety. Design changes to cell shape and cell size also affect safety. Selection of the electrode materials and electrolyte affect performance and safety.
Typically, designers are able to make simple cell performance and energy density estimates based on projections from data collected in lab cells. However, it has heretofore not been possible to reliably predict safety test results of practical cells (e.g., full-scale consumer batteries) based on test results at the lab scale. In order to conduct reliable safety studies, prototyping of a potential product in actual cell hardware, followed by extensive testing, is presently necessary. Moreover, large quantities of electrode materials must be produced in order to properly construct prototype cells for safety testing and evaluation. Conventional cell/battery design and development techniques typically require the production and availability of 10 kilograms or more of sample electrode material. Those skilled in the art readily appreciate that designing, developing, and testing electrochemical cells and batteries, particularly those having a custom, non-industry standard configuration, using conventional approaches is extremely time consuming and costly.
There is a need in the battery manufacturing industry for systems and methods that assist in the design of electrochemical cells and batteries of varying technologies, and which require the production of small quantities of sample electrode materials. There exists a further need for such systems and methods that eliminate the present need to construct full-scale cell/battery prototypes in order to fully evaluate the safety aspects of a given cell/battery design. The present invention fulfills these and other needs.
The present invention is directed to methods and apparatuses for characterizing electrochemical cell components and for characterizing a response of an electrochemical cell to a specified operating condition. According to one embodiment of the present invention, characterizing electrochemical cell components involves preparing a sample of an electrode material in contact with an electrolyte. Self-heating, power-temperature or power-time data is obtained for the sample using a calorimetry technique, such as by use of an accelerating rate calorimetry technique or a differential scanning calorimetry technique, for example. Obtaining self-heating data, for example, may involve obtaining temperature versus time data of the sample during substantially adiabatic reaction.
A power function is developed for the sample using the self-heating, power-temperature or power-time data. The power function is representative of thermal power per unit mass of the sample as a function of temperature and amount of reactant remaining from a reaction of the sample electrode material and electrolyte.
In general, preparing the electrode material sample involves preparing the sample using less than about 100 grams of the electrode material. According to one embodiment, preparing the electrode material sample involves preparing the sample using between about 1 gram and about 10 grams of the electrode material. In another embodiment, preparing the electrode material sample involves preparing the sample using between about 1 milligram and about 1 gram of the electrode material. Improvements in calorimetry techniques may provide for the development of power functions for electrode material samples using nanograms of the electrode material samples. The electrode material may be a cathode material or an anode material. The electrode material may, for example, include lithium.
In accordance with another embodiment, characterizing electrochemical cell components involves preparing a first sample of a cathode material in contact with an electrolyte and preparing a second sample of an anode material in contact with the electrolyte. First and second self-heating, power-temperature or power-time data are obtained for the first and second samples, respectively, using a calorimetry technique. A first power function for the first sample and a second power function for the second sample are developed using the first and second self-heating, power-temperature or power-time data, respectively. The first power function characterizes a reaction between the cathode material and the electrolyte in terms of thermal power per unit mass of the cathode sample material, and the second power function characterizes a reaction between the anode material and the electrolyte in terms of thermal power per unit mass of the anode sample material.
Preparing the first sample typically involves preparing the first sample using less than about 100 grams of the cathode material, and preparing the second sample typically involves preparing the second sample using less than about 100 grams of the anode material. According to one embodiment, preparing the first sample involves preparing the first sample using between about 1 and 10 grams of the cathode material, and preparing the second sample involves preparing the second sample using between about 1 and 10 grams of the anode material. In another embodiment, preparing the first sample involves preparing the first sample using between about 1 milligram and about 1 gram of the cathode material, and preparing the second sample involves preparing the second sample using between about 1 milligram and about 1 gram of the anode material. The cathode and anode material may each include lithium. The calorimetry technique employed may be an accelerating rate calorimetry technique or a differential scanning calorimetry technique.
According to another embodiment of the present invention, characterizing an electrochemical cell involves defining one or more physical parameters of the electrochemical cell. A first power function characterizing a reaction between a cathode and an electrolyte in terms of thermal power per unit mass of cathode material is provided. Also provided is a second power function characterizing a reaction between an anode and the electrolyte in terms of thermal power per unit mass of anode material. A response of the cell to a specified operating condition is predicted using the first and second power functions and the physical parameters of the electrochemical cell. In one embodiment, characterizing the electrochemical cell in this manner is implemented using a computer and user-interface coupled to the computer.
Defining one or more physical parameters of the cell may further involve adjusting the physical parameters of the cell. Predicting the response of the cell, in this case, involves predicting the response of the cell using the first and second power functions and the adjusted physical parameters of the cell.
Defining one or more physical parameters of the cell may also involve receiving user input data representative of physical parameters of the cell. Receiving user input data may further involve presenting to a user an input field corresponding to each physical parameter of the cell and receiving input data from the user in each of the input fields. Defining one or more physical parameters of the cell may also involve receiving physical parameters of the cell electronically, such as from an external local or remote host processor.
Defining one or more physical parameters of the cell may further involve defining one or more physical parameters for each of an anode and a cathode of the cell. Defining physical parameters for each of the anode and cathode of the cell may further involve adjusting the physical parameters of one or both of the anode and cathode. Predicting the response of the cell in this case further involves predicting the response of the cell using the first and second power functions and the adjusted physical parameters of one or both of the anode and cathode.
The specified operating condition may, for example, include a condition of constant or varying ambient temperature, a condition of a constant or varying current applied to the cell, a condition of an external short-circuit connected to the cell or a condition of a short-circuit internal to the cell.
A system for characterizing an electrochemical cell, in accordance with yet another embodiment of the present invention, includes a processor and a user-interface coupled to the processor. The user-interface includes an input device operable by a user for entering one or more physical parameters of the electrochemical cell. The system further includes memory coupled to the processor. The memory stores a cathode power function characterizing a reaction between a cathode and an electrolyte in terms of thermal power per unit mass of cathode material, and further stores an anode power function characterizing a reaction between an anode and the electrolyte in terms of thermal power per unit mass of anode material. The processor computes a response of an electrochemical cell to a specified operating condition using the cathode and anode power functions and the physical parameters of the electrochemical cell.
The input device is further operable by the user to enter physical parameters of an anode and a cathode of the cell. The processor, according to this embodiment, computes the response of the electrochemical cell to a specified operating condition using the cathode and anode power functions and the user-entered physical parameters of the anode and cathode of the electrochemical cell. The input device is also operable by the user to adjust physical parameters of the cell, and the processor further computes the response of the electrochemical cell to a specified operating condition using the cathode and anode power functions and the adjusted physical parameters of the electrochemical cell. A user may also use the input device to adjust physical parameters of an anode and a cathode of the cell, and the processor computes the response of the electrochemical cell to the specified operating condition using the cathode and anode power functions and the adjusted physical parameters of the anode and cathode of the electrochemical cell.
The system may further include a display. The input device is operable by the user for entering physical parameters of the electrochemical cell into input fields presented on the display. Physical parameters of an anode and a cathode of the electrochemical cell may also be entered using the input device into input fields presented on the display. The system may further include a calorimeter system coupled to the processor. The calorimeter system may include an accelerating rate calorimeter or a differential scanning calorimeter.
The memory of the system may be situated proximate the processor, situated remotely from the processor or distributed at locations local to and/or remote from the processor. The memory that stores the anode and cathode power functions, for example, may be partially or completely situated remotely from the processor. Power functions developed for a number of electrode/electrolyte combinations may be stored in a database or in libraries. Power functions and libraries of power functions may be accessed via a network connection.
Characterizing electrochemical cell components in accordance with another embodiment of the present invention involves defining one or more physical parameters of an electrochemical cell, and characterizing a reaction between a cathode and an electrolyte in terms of thermal power per unit mass of cathode material by defining a first power function. A reaction between an anode and the electrolyte in terms of thermal power per unit mass of anode material is also characterized by defining a second power function. The response of the cell to a specified operating condition is estimated using the first and second power functions and the physical parameters of the electrochemical cell.
Characterizing the respective cathode/electrolyte and anode/electrolyte reactions according to this embodiment involves modeling the respective reactions assuming an autocatalytic reaction mechanism. The first power function, Pc, associated with the cathode/electrolyte reaction may be characterized by the following equations:                     ⅆ        u                    ⅆ        t              =          k      ⁢              xe2x80x83            ⁢              (                  1          -          u                )            ⁢              xe2x80x83            ⁢              (                  β          +                      u            0.5                          )                                ⅆ        T                    ⅆ        t              =                  h                  C          tot          xe2x80x2                    *                        ⅆ          u                          ⅆ          t                    
where, u represents a dimensionless fractional degree of conversion, k represents a reaction rate constant defined by k=xcex3 exp(xe2x88x92Ea/kbT), xcex3 represents a frequency factor expressed in terms of minutesxe2x88x921, Ea represents activation energy, kb represents Boltzmann""s constant, T represents a temperature of the cell, xcex2 represents a dimensionless parameter of autocatalysis, h represents total heat which can be evolved by a sample of cathode material during reaction expressed in terms of Joules, Cxe2x80x2tot represents a total heat capacity of the reactant and a sample calorimeter bomb expressed in terms of J/K, and H represents total heat generated by the cathode/electrolyte reaction per gram of cathode material.
The second power function, Pa, associated with a lithium intercalated carbon anode/electrolyte reaction, may be characterized by:       P    B    =                    H        2            ⁢              xe2x80x83            ⁢              "LeftBracketingBar"                              ⅆ                          x              2                                            ⅆ            t                          "RightBracketingBar"              +                  H        1            ⁢              "LeftBracketingBar"                              ⅆ                          x              1                                            ⅆ            t                          "RightBracketingBar"            
where,                     ⅆ                  x          2                            ⅆ        t              =                  -                  γ          2                    ⁢              xe2x80x83            ⁢              exp                                            -                              E                2                                      /                          k              b                                ⁢                      xe2x80x83                    ⁢          T                    ⁢              xe2x80x83            ⁢              x        2        0.5                                ⅆ                  x          2                            ⅆ        t              =                  -                  γ          2                    ⁢              xe2x80x83            ⁢              exp                                            -                              E                2                                      /                          k              b                                ⁢                      xe2x80x83                    ⁢          T                    ⁢              xe2x80x83            ⁢              x        1            ⁢              xe2x80x83            ⁢              exp                              -                          (                                                (                                                            x                                              3                        ⁢                        o                                                              +                                          x                                              2                        ⁢                        o                                                                              )                                +                                  f                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  x                                                  1                          ⁢                          o                                                                    -                                              x                        1                                                              )                                                              )                                /                      (                                          x                                  3                  ⁢                  o                                            +                              x                                  2                  ⁢                  o                                                      )                              ⁢              xe2x80x83            ⁢      and                          ⅆ                  x          3                            ⅆ        t              =                            ⅆ                      x            1                                    ⅆ          t                    -                        ⅆ                      x            2                                    ⅆ          t                    
and where, x1 represents an amount of type 1 lithium measured as x in LixC6, x2 is an amount of type 2 lithium, measured per six carbons, and x3 is an amount of type 3 lithium, measured per six carbons, x1o, x2o, and x3o are initial amounts of lithium after electrochemical discharge and before heating, E1 and E2 are activation energies, and xcex31 and xcex32 are frequency factors, f is a constant of proportionality that governs how fast the layer of reaction products on the surface of the carbon grows as type 1 lithium is converted to type 3 lithium, and H1 and H2 are the heat per gram of carbon due to the changes xcex94x1=xe2x88x921 and xcex94x2=xe2x88x921, respectively.
Characterizing the cathode/electrolyte reaction may involve characterizing the cathode/electrolyte reaction using less than about 100 grams of cathode material, and characterizing the anode/electrolyte reaction may involve characterizing the anode/electrolyte reaction using less than about 100 grams of anode material. According to one embodiment, characterizing the cathodelelectrolyte reaction involves characterizing the cathode/electrolyte reaction using between about 1 and 10 grams of cathode material, and characterizing the anode/electrolyte reaction involves characterizing the anode/electrolyte reaction using between about 1 and 10 grams of anode material. In another embodiment, characterizing the cathode/electrolyte reaction involves characterizing the cathode/electrolyte reaction using between about 1 milligram and about 1 gram of cathode material, and characterizing the anode/electrolyte reaction involves characterizing the anode/electrolyte reaction using between about 1 milligram and about 1 gram of anode material. The cathode and anode material may each include lithium.
Characterizing the first and second power functions may involve obtaining temperature versus time data, power versus temperature data or power versus time data for each of the cathode/electrolyte and anode/electrolyte reactions. The first and second power functions are preferably characterized using a calorimetry technique, such as an accelerating rate calorimetry technique or a differential scanning calorimetry technique. The specified operating condition may involve a condition of constant or varying ambient temperature, a condition of a constant or varying current applied to the cell, a condition of an external short-circuit connected to the cell or a condition of a short-circuit internal to the cell.
In accordance with yet another embodiment, a computer readable medium embodying program instructions for characterizing electrochemical cell components is provided. The computer medium embodies program instructions executable by a processor that characterize a reaction between a cathode and an electrolyte in terms of thermal power per unit mass of cathode material by defining a first power function, and further characterize a reaction between an anode and the electrolyte in terms of thermal power per unit mass of anode material by defining a second power function. The program instructions executable by the processor further provide for defining one or more physical parameters of the electrochemical cell, and predicting a response of the cell to a specified operating condition using the first and second power functions and the physical parameters of the electrochemical cell.
According to this embodiment, characterizing the respective cathode/electrolyte and anode/electrolyte reactions involves modeling the respective reactions assuming an autocatalytic reaction mechanism. The first power function, Pc, associated with the cathode/electrolyte reaction, and the second power function, Pa, associated with the anode/electrolyte reaction, may be respectively computed using the equations provided hereinabove.
Defining one or more physical parameters of the cell may further involve adjusting the physical parameters of the cell, and predicting the response of the cell further involves predicting the response of the cell using the first and second power functions and the adjusted physical parameters of the cell. Defining one or more physical parameters of the cell may also involve receiving user input data representative of physical parameters of the cell. Receiving user input data further may involve presenting to a user an input field corresponding to each physical parameter of the cell, and receiving input data from the user in each of the input fields. Defining one or more physical parameters of the cell may further involve receiving physical parameters of the cell electronically.
One or more physical parameters of the cell may be defined for each of an anode and a cathode of the cell. Defining physical parameters for each of the anode and cathode of the cell may involve adjusting the physical parameters of one or both of the anode and cathode, and predicting the response of the cell involves predicting the response of the cell using the first and second power functions and the adjusted physical parameters of one or both of the anode and cathode. The specified operating condition may involve a condition of constant or varying ambient temperature, a condition of a constant or varying current applied to the cell, a condition of an external short-circuit connected to the cell or a condition of a short-circuit internal to the cell.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.